Computer hard drives store data by affecting the magnetic field of memory cells on a hard drive disk. The stored data is read by passing a read head sensor above a memory cell to respond to, and thus detect, the orientation of the magnetic field in the memory cell. The smaller the memory cells on the hard drive disk, the more densely they can be packed, increasing the density of data storage possible on a hard drive disk.
However, making smaller memory cells is not all that is required to increase data density storage capacity. Increasingly smaller memory cells require increasingly smaller read sensors, particularly read sensors with a narrow track width, in order to be responsive to the magnetic field of a single memory cell.
Currently there is no commercial lithography tool than can provide read sensors with line widths less than about 30 nm.